In order to improve a bead part of a tire for heavy-duty vehicles to reduce a heat generation thereof, various approaches such as changing a blend amount of carbon black and carbon black species, and/or applying a dispersion improving agent have so far been tried. The heat generation can be improved by the above conventional methods, but the durability and the crack resistance were found to be lowered, and tires produced by the conventional methods were unsatisfactory for applications in heavy-duty vehicles used under severe conditions.
In particular, when the carbon black species is intended to be changed, various carbon blacks including commercially available products produced by modification of furnaces are available. However, the properties of carbon blacks cannot necessarily be selected as desired, and it is not easy to select the carbon black species which meet the purposes by trial and error.
In the above situations, if carbon blacks having properties which originate in two or more carbon black species can be used, expected are the advantages that the choice of carbon blacks is broadened and that the intended performances are readily estimated by modifying a blend ratio thereof.
It is disclosed in patent document 1 that a product referred to as hard carbon black having a nitrogen adsorption specific surface area of 60 m2/g or more and a product referred to as soft carbon black having a nitrogen adsorption specific surface area of 50 m2/g or less are used in a mixture. Selection from two divisions in the nitrogen adsorption specific surface area is shown therein, however, relationship between the respective divisions is not shown.
It is disclosed in patent document 2 that two kinds of carbon black of a large particle diameter having an iodine adsorbing No. of less than 115 and carbon black of a small particle diameter having an iodine adsorbing No. of exceeding 115 are used in a mixture, but selection from two divisions in which 115 is set to a boundary in the iodine adsorbing No. as a standard of a particle diameter is shown in the above document, however, relationship between the respective divisions is not shown. Similarly, divisions prescribed by DBP No. and iodine adsorbing No. are disclosed as well therein, however, selection from two divisions in which 110 is set to a boundary in the DBP No. is shown also in the above case, and relationship between the respective divisions is not shown therein.
It is disclosed in patent document 3 that two kinds of carbon blacks which are prescribed by a nitrogen adsorption specific surface area and dibutyl phthalate absorption are used in a mixture, however, it is not shown therein what relationship has to be present between physical property values relevant to the particle sizes of the above carbon blacks. Two kinds of the specific carbon blacks are merely used, and the respective carbon blacks are selected based on a division of whether the above physical property values are larger or smaller than a certain threshold value. Also, an effect caused by the other specific components is more predominant than an effect caused by mixing the carbon blacks, and an effect caused by a difference between the blend ratios of the two kinds of the carbon blacks is not clear.
It is disclosed in patent document 4 that two kinds of carbon blacks in which a relationship between a cetyltrimethylammonium bromide adsorption specific surface area and a dibutyl phthalate absorption is prescribed are used in a mixture, however, a relationship between the physical property values of the two kinds of the carbon blacks is not clarified.
Also, a method for mixing carbon blacks based on more basic physical property values such as a particle diameter or a particle size of carbon blacks is not disclosed in any of the patent documents 1 to 4.